Flak is a type of projectile mainly used in anti-air warfare. Flak burst in mid air causing it to hit several enemies at once. It does little damage per shot, but the fact that it can hit several targets at once makes up for this. Flak is best used against enemy Gunships, transports and Tech 1 units, due to their low health.
Flak is great against transports as it can destroy it's cargo if not the whole transport easily, however, like any non-tech 3 AA, flak has a hard time hitting tech 3 air superiority fighters as they are too fast and manueverable. If you are cybran, you can use your transport to send a deceiver and engineers to build some flak near an undefended air factory to destroy it's creations as your engineer captures it.